What's the best way to get a book published?

I wrote a book when I was 15 and tried to get it published, but because of my age and the fact I was living in the Middle East, I was met with many responses along the line of ‘get in touch when you’re 18 or back in the UK’.
I’m 29 now and I’m in the middle of re-writing the book using my more developed adult literacy skills and I want to try to get it published again.
Agents or publishers? Anyone have any experience in this area?